<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>

<body>
    <button>点击发送</button>
    <!-- <button>点击取消</button> -->
    <script>
        const btns = document.querySelectorAll('button');
        let x = null;
        // 变量标识符
        let isSending = false;//是否正在发送AJAX请求
        btns[0].onclick = function () {
            if (isSending) x.abort();//如果正在发送，则取消该请求，创建一个新的请求
            x = new XMLHttpRequest();
            isSending = true;
            x.open('GET', 'http://127.0.0.1:8000/delay');
            x.send();
            x.onreadystatechange = function () {
                if (x.readyState === 4) {
                    //修改标识变量
                    isSending = false;
                }

            }
        }
        // abort方法 取消请求
        btns[1].onclick = function () {
            x.abort();

        }
    </script>
</body>

</html>